The rotor of the electrical machine is guided in rotation in a bearing, generally a roller or ball bearing, carried by an annular cover that closes the stator of the electrical machine and that serves to secure it to the casing of the accessory box.
The electrical machine is characterized by speeds of rotation that are high and by loads that are low, thereby leading to dynamic phenomena that can in turn lead to damage to the bearing.
In order to avoid or at least reduce that drawback, a cover is used that is made of titanium because it has a low coefficient of thermal expansion, thus limiting differential thermal expansion between the outer ring of the bearing and the portion of the cover in which said ring is mounted.
This avoids creating significant clearance in the bearing, where such clearance tends to increase the unbalance of the rotor and gives rise to rapid wear and deterioration of the bearing.
In operation, the electrical machine leads to a large amount of heat being dissipated in the winding of its stator, and this heat must be evacuated by conduction into the casing of the box, e.g. made of aluminum and thus constituting a good conductor of heat.
Nevertheless, the stator cover of the electrical machine impedes this evacuation of heat since titanium is a poor conductor of heat (its coefficient of thermal conduction is about one-twentieth that of aluminum). This leads to heat accumulating and to a significant rise of temperature in the electrical machine, thus leading to a risk of the oil contained in the machine coking and to excess heating of the stator which loses its mechanical properties.